1. Field of the Invention
This invention relates to a stabilized ethylene-tetrafluoroethylene copolymer composition which is stabilized against thermal degradation, and more particularly, this invention relates to an ethylene-tetrafluoroethylene copolymer which is stabilized against thermal degradation by addition of a copper compound. This invention also relates to a process for preparing the stabilized compositions.
2. Description of the Prior Art
It is known that ethylene-tetrafluoroethylene copolymers have excellent thermal, chemical, electrical and mechanical properties and are melt-processable. These copolymers are known to be heat resistant thermoplastic resins which have a fluid initiation temperature or a melting point of 260.degree. to 300.degree. C. However, conventional ethylene-tetrafluoroethylene copolymers thermally deteriorate and become colored, brittle and foamed when heated to a temperature higher than the melting point, such as higher than 300.degree. C, for a long period of time. Accordingly, it is necessary to prevent the thermal deterioration of ethylene-tetrafluoroethylene copolymers by preventing a rise of temperature and decreasing the residence time of the thermoplastic resin in a molding apparatus during the conventional operation of injection molding and extrusion molding processes. The thermal deterioration of ethylene-tetrafluoroethylene copolymers worsens in dependence upon the rise in the temperature, especially in an air atmosphere.
On the other hand, in the powder coating method for spray-coating ethylene-tetrafluoroethylene copolymer powders onto a metal substrate, it has been known to employ
(1) A METHOD OF PREHEATING THE SUBSTRATE AT A TEMPERATURE HIGHER THAN THE MELTING POINT OF THE COPOLYMER TO MELT AND ADHERE THE COPOLYMER POWDER ON IT; AND
(2) A METHOD OF ELECTROSTATICALLY COATING THE COPOLYMER POWDER ON THE SUBSTRATE AND MELTING IT.
For example, in order to form a coated film having a thickness of 600 micrometers, as required for an anticorrosive lining, by a powder coating method, it is necessary to preheat the substrate to a temperature higher than 340.degree. C, and
(1) IT IS NECESSARY TO REPEAT A BAKING OPERATION SEVERAL TIMES IN THE METHOD
(2) BECAUSE THE MAXIMUM THICKNESS OF LAYER RESULTING FROM ONE COATING IS IN THE RANGE OF ABOUT 150 TO 200 MICROMETERS.
In both methods, it is possible to attain a thick lining when the operation is carried out on a small substrate using a small size test oven while precisely controlling the temperature. However, it is difficult to obtain the desired coated film because of thermal degradation of the resin caused by non-uniform distribution of temperature in the oven and non-uniform thickness of the substrate when the operation is carried out on a large substrate in an industrial oven having a length of several meters.
The inventors have proposed a thermal stabilizing method which involves combining a small amount of .alpha.-alumina with the ethylene-tetrafluoroethylene copolymer (U.S. Pat. No. 3,893,971). Although remarkable effects for stabilizing against thermal degradation could be attained by this thermal stabilizing method using .alpha.-alumina at relatively low temperatures of about 310.degree. C, it has been difficult to attain satisfactory thermal stabilization at higher temperatures such as about 340.degree. C. Accordingly, it has been difficult to achieve smoothly and advantageously a thick lining in the powder coating method. These disadvantages cause difficulty when the ethylene, tetrafluoroethylene copolymer remains at a high molding temperature for a long residence time in the molding apparatus, when it is locally overheated or when it is molded under a high temperature atmosphere.